1. Field of the Invention
The present invention relates to an improved, highly reliable pin junction photovoltaic device which stably and continuously exhibits an improved photoelectric conversion efficiency without being deteriorated even upon repeated use over a long period of time. More particularly, the present invention relates to a pin junction photovoltaic device having a buffer layer positioned between a p-type semiconductor layer and an i-type semiconductor layer and another buffer layer positioned between said i-type semiconductor layer and an n-type semiconductor layer, wherein said i-type semiconductor layer is formed of an amorphous silicon germanium (that is, a-SiGe) material and has a maximal point for the content of the germanium atoms contained therein. The pin junction photovoltaic element is effectively usable as a solar cell or a photosensor.
2. Related Background Art
There are presently a variety of photovoltaic devices which have been put into practical use as solar cells for power supplies and also as photosensors in image readers.
Now, at the present time, solar cells mostly have been used for power sources of consumer electronic apparatus such as electronic calculators, watches, etc. However, solar cells are expected to be a future power generation source since they supply electric power without causing CO.sub.2 buildup as in the case of oil fired power generation.
Solar cells are based on the technology of utilizing photoelectromotive force generated in a semiconductor active region having a pn junction. Such semiconductor active region with a pn junction is generally formed by using a silicon-containing semiconductor material or a germanium-containing semiconductor material. In the semiconductor active region, light such as sunlight is absorbed, the absorbed light generates photocarriers including electrons and holes, and the photocarriers are separated by the action of an internal electric field of the pn junction, whereby photoelectromotive force is outputted.
Of the presently known solar cells, a solar cell made of a single crystalline silicon material has been found to be highly reliable and high in photoelectric conversion efficiency (this solar cell will be hereinafter called "single crystal silicon solar cell"). However, there are disadvantages of the single crystal silicon solar cell as will be described in the following. That is, it is costly since it is produced by way of the so-called semiconductor wafer process. And the single crystalline silicon material of which the single crystal silicon solar cell is constituted is relatively small in light absorbance and because of this, it is necessary for the single crystal silicon solar cell to have a thick semiconductor active region with a thickness of more than 50 .mu.m in order to facilitate its function absorb light such as sunlight. In addition, the single crystalline silicon material of the single crystal silicon solar cell is of about 1.1 eV in band gap and because of this, short wavelength energy components of the sunlight spectrum are not utilized for photoelectric conversion in the single crystal silicon solar cell. Further in addition, it is extremely difficult for the single crystal silicon solar cell to be of a large area because there is a limit for the size of silicon wafers that can be produced because of the requirement for growing a single crystal.
There are known a number of solar cells made of a polycrystalline silicon material (this solar cell will be hereinafter called "polycrystal silicon solar cell"). The polycrystal silicon solar cell is advantageous in that is can be produced at a cost which is lower than that of the single crystal silicon solar cell. However, as for the polycrystal silicon solar cell, there are disadvantages similar to those in the case of the single crystal silicon solar cell. Particularly, the polycrystalline silicon material of which the polycrystal silicon solar cell is constituted is relatively low in light absorbance and because of this, it is necessary for the polycrystal silicon solar cell to have a thick semiconductor active region as in the case of the single crystal silicon solar cell. In addition, the semiconductor active region of the polycrystal silicon solar cell contains grain boundaries and because of this, the polycrystal silicon solar cell is not satisfactory in terms of solar cell characteristics.
In view of the above, solar cells made of an amorphous silicon material (that is, a-Si; this solar cell will be hereinafter called "amorphous silicon solar cell") have been spotlighted because of their various advantages as will be described in the following. That is, their constituent semiconductor film can be relatively easily formed in a large area, thinned as desired, and formed on a selected substrate. In addition to these advantages, there are also other advantages in that a large area solar cell can be produced on an industrial scale, and thus, it is possible to provide an amorphous silicon solar cell at a reasonable cost.
There is, however, a disadvantage of such an amorphous silicon solar cell in that its photoelectric conversion efficiency is inferior to that of the above-mentioned single crystal silicon solar cell and is not sufficient for the amorphous silicon solar cell to be used as a daily power supply source.
In order to improve such a disadvantage of the amorphous silicon solar cell, U.S. Pat. No. 4,377,723 or Japanese Unexamined Patent Publication No. 125680/1980 proposes a technique of improving the open-circuit voltage (Voc) of an amorphous silicon solar cell by stacking a plurality of photovoltaic cell units each having a pn or pin junction.
Besides the above proposal, U.S. Pat. No. 4,542,256, U.S. Pat. No. 4,816,082, and U.S. Pat. No. 4,816,082 propose a technique of improving the light absorbance of an amorphous silicon solar cell by varying the band gap of each of the constituent semiconductor layers. Particularly, said U.S. Pat. No. 4,542,256 describes a technique of improving the open-circuit voltage (Voc) and short-circuit current (Jsc) of a pin junction solar cell by disposing a layer (that is a so-called intermediate layer) having a continuously graded band gap with an affinity gradient either at the interface between the p-type semiconductor layer and the i-type semiconductor layer or at the interface between the n-type semiconductor layer and the i-type semiconductor layer.
However, neither of these proposals is sufficient in terms of providing a high enough photoelectric conversion efficiency in a pin junction amorphous silicon photovoltaic cell.
FIG. 24 is a schematic diagram illustrating the profile of the energy band gap in the photovoltaic cell described in the above-mentioned U.S. Pat. No. 4,542,256 which has such a graded band gap layer as above described.
In FIG. 24, reference numeral 1 indicates an n-type amorphous silicon germanium semiconductor layer (that is, an n-type a-SiGe semiconductor layer), reference numeral 2 indicates a non-doped (i-type) amorphous silicon germanium semiconductor layer (that is, an i-type a-SiGe semiconductor layer), reference numeral 3 indicates a graded band gap layer comprised of a non-doped a-SiGe semiconductor material in which the composition ratio between the Si atoms and the Ge atoms is varied in the thickness direction, and reference numeral 4 indicates a p-type amorphous silicon semiconductor layer (that is, a p-type a-Si semiconductor layer). Particularly, the composition ratio between the Si atoms and the Ge atoms constituting the graded band gap layer 3 is designed such that the layer contains Ge atoms in an amount of 20 atomic % on the side of the i-type a-SiGe semiconductor layer but does not contain Ge atoms on the side of the p-type a-Si semiconductor layer 4.
FIG. 25 is a schematic diagram illustrating the profile of the energy band gap in the photovoltaic cell described in the above-mentioned U.S. Pat. No. 4,816,082.
In FIG. 25, reference numeral 1' indicates an n-type semiconductor layer comprised of a microcrystalline silicon semiconductor material (that is, .mu.c-Si), reference numeral 2' indicates an i-type semiconductor layer comprised of a non-doped a-SiGe semiconductor material in which the content of the Ge atoms is continuously varied from 0 atomic % to 30 atomic % in the thickness direction, reference numeral 3' indicates an i-type semiconductor layer comprised of a non-doped a-SiGe in which the content of the Ge atoms is continuously varied from 30 atomic % to 0 atomic % in the thickness direction, reference numeral 5 indicates an i-type a-Si semiconductor layer, and reference numeral 4' indicates a p-type .mu.c-Si semiconductor layer.
As apparent from FIG. 25, in the profile of the energy band gap in the photovoltaic cell described in said U.S. Pat. No. 4,816,082, most of the i-type semiconductor layer regions are designed to have a graded band gap while having a layer region with a minimum band gap at a given position.
FIG. 26 is a schematic diagram illustrating the profile of the energy band gap in a photovoltaic element having an i-type semiconductor layer with a plurality of layer regions having different graded band gaps which has been previously proposed by two of the three inventors of the instant invention together with three others (see U.S. Pat. No. 5,252,142).
In FIG. 26, reference numeral 1' indicates an n-type .mu.c-Si layer, reference numeral 3' indicates an i-type a-SiGe semiconductor layer in which the content of the Ge atoms is continuously varied from 30 atomic % to 0 atomic % in the thickness direction (this means that the i-type a-SiGe semiconductor layer has a graded band gap), reference numeral 5' indicates an i-type semiconductor layer, and reference numeral 4' indicates a p-type semiconductor layer formed of a p-type microcrystalline silicon germanium film (that is, a p-type uc-SiGe film). The photovoltaic element shown in FIG. 26 further contains an i-type a-SiGe semiconductor layer 6 with a graded band gap and another i-type a-SiGe semiconductor layer 7 with a graded band gap which is different from that of the i-type a-SiGe semiconductor layer 6 between the n-type semiconductor layer 1' and the i-type semiconductor layer 3'. This configuration is thus different from that of the photovoltaic element shown in FIG. 25.
Each of the proposals mentioned in FIGS. 24 to 26 is of the technique wherein separation of photocarriers due to their drift is facilitated by the action of a layer with a continuously graded band gap, whereby the photocarriers are prevented from recombining.
The present inventors have studied these proposals. As a result, it has been found that the technique according to any of these proposals is effective in improving the initial photoelectric conversion efficiency of an amorphous silicon photovoltaic cell to a certain extent but there still remains a problem in that when the photovoltaic cell is continuously irradiated with light over a long period of time, the photoelectric conversion efficiency is liable to deteriorate, as in the case of an amorphous silicon photovoltaic cell without having such a graded band gap layer as above described.